Various techniques are known for conducting non-destructive inspection of structures having a surface area. In one known technique, monochromatic, phase coherent electromagnetic radiation in the 5–50 GHz frequency range (i.e., microwaves) is used for nondestructive testing of dielectric materials. A portion of the impinged beam is combined with the signal reflected by the specimen being inspected. The signals combine to produce an interference pattern, a pattern that changes as the specimen changes, or as the position of the specimen changes relative to that of the detector.
In another known technique, a near-field microwave microscope including a dielectric resonator is used. The microwave microscope has a resonant slit in the conductive end of a microwave waveguide that forms a probe tip. A short dielectric rod is fit into the microwave waveguide near its conductive end. A longer dielectric rod is placed in back of the short dielectric rod with a small gap between the two rods. The length of the shorter rod and the size of the gap are chosen to form a dielectric resonator at the microwave frequency adjacent to the probe tip.
Yet another known testing technique uses a near-field sensor including circuitry which removes variation in the “standoff distance” (i.e., the distance of the sensor from the inspected object) as a factor in the inspection system readings. An original output voltage which varies linearly according to the standoff distance is modified and added to a counterbalancing output voltage which equivalently but oppositely varies linearly according to the standoff distance, resulting in a constant output voltage regardless of the standoff distance.